exports.authenticate = function (req, res, cb) {
  var err = null,
    cookieName = 'linkedin_oauth_' + apiKey,
    cookie = req.cookies[cookieName];

  if (cookie) {
    var credentials = JSON.parse(cookie);

    if (credentials.signature_version === '1') {
      var sigOrder = credentials.signature_order;
      if (sigOrder && util.isArray(sigOrder)) {
        var baseString = '';
        var i, key;

        for (i = 0; i < sigOrder.length; i++) {
          key = sigOrder[i];
          if (credentials[key]) {
            baseString += credentials[key];
          } else {
            err = 'Invalid Linkedin key in signature order';
            break;
          }
        }

        if (!err) {
          var hmac = crypto.createHmac('sha1', apiSecretKey);
          var sig = hmac.update(baseString).digest('base64');
          if (sig === credentials.signature) {
            var ao = new OAuth.OAuth('', accessTokenUrl, apiKey, apiSecretKey,
              '1.0a',
              null, credentials.signature_method);

            var params = {
              xoauth_oauth2_access_token: credentials.access_token
            };

            ao.post(accessTokenUrl, null, null, params, null,
              function (results, data) {
                if (data) {
                  var r = querystring.parse(data);
                  r.member_id = credentials.member_id;
                  cb(null, r);
                  return;
                } else {
                  err = 'Likedin access token not returned';
                }
              });
          } else {
            err = 'Linkedin signature mismatch';
          }
        }
      } else {
        err = 'Invalid Linkedin signature order';
      }
    } else {
      err = 'Invalid Linkedin cookie version: ' + credentials.signature_version;
    }
  } else {
    err = 'Linkedin Cookie not set';
  }

  if (err) {
    cb(err, {});
  }
};